关键词: 土壤孔隙含水量, N₂O, 硝化作用, 反硝化作用, 稳定同位素, 同位素位嗜值

Abstract: 【Objective】 The objective of this paper is to understand the contribution of microbial processes to N₂O production and its changing rules under different water contents to provide a theoretical basis for reducing agricultural N₂O emissions. 【Method】A microcosm experiment was performed to investigate the effects of different water-filled pore space on N₂O emissions and isotopic signatures (δ¹⁵N^bulk, δ¹⁸O and nitrogen isotopomer site preference of N₂O) of soil at Shunyi district, Beijing. The study combined stable isotope technique and gas inhibitor method to analyze N₂O flux and its isotope signatures that emitted from soil. The experiment was set up three different water content levels, including 67%, 80% and 95% WFPS, and with three inhibitor levels, (without C₂H₂, with 0.1% (V/V) C₂H₂ and with 10% (V/V) C₂H₂). After two hours incubation, the soil samples were collected to determine the concentrations of NH+ 4-N and NO- 3-N. The gas samples were collected to determine the isotope signatures, and the two end-members mixing model was applied to quantify the respective contributions of microbial processes to N₂O production. 【Result】According to the incubation of the soil, the weighted average N₂O flux of 95%, 80% and 67% WFPS were 1.17, 0.27 and 0.08 mgN·kg^-1·d^-1, respectively, and the N₂O emissions of 95% WFPS were significantly higher than that of both 80% and 67% WFPS, as well as the N₂O emissions of 80% WFPS were significantly higher than that of 67% WFPS. The cumulative emissions of (N₂O+N₂) in 95%, 80% and 67% WFPS were 18.05%, 5.27%, and 1.24% of initial mineral nitrogen, respectively, over the entire incubation period. The cumulative emissions of (N₂O+N₂) were 19.61, 5.72 and 1.35 mgN·kg^-1, respectively; the initial content of NH+ 4-N+NO- 3-N was 108.62 mgN·kg^-1. Compared with 67% WFPS, the cumulative (N₂O+N₂) emissions of 95% and 80% WFPS increased 13.53 and 3.24 times, respectively. The cumulative emissions of (N₂O+N₂) in 95% WFPS was 2.43 times greater than that of 80% WFPS. The values of reduced NH+ 4-N+NO- 3-N as gaseous nitrogen increased with the increase of the water content. The weighted average δ¹⁵N^bulk values varied from -42.93‰ to -4.07‰, and the higher level of soil water content showed significantly higher N₂O emissions. 10% (V/V) C₂H₂ would inhibit the reduction of N₂O to N₂. The δ¹⁸O values with 10% (V/V) C₂H₂ were significantly smaller than that of with 0.1% (V/V) C₂H₂ in three water content levels. And the ratio of N₂O/(N₂O+N₂) reduced with the increase of soil moisture. Multiple N₂O processes occurred simultaneously in all treatments. The values of SP increased during the initial four days and then decreased gradually with incubation time. The SP values of 67%WFPS treatment at the first two days ranged from 6.74‰ to 12.04‰, and the contribution of denitrification to N₂O production was from 56.36% to 66.15%, suggesting that denitrification was the dominant microbial process, then the contribution of nitrification (55.78%-100%) to N₂O production became greater. The weighted average SP value was 10.26‰ in 80% WFPS treatment, indicating denitrification (40.90%-74.04%) was the major N₂O production process. There were larger SP values in 95% WFPS treatment with 10% (V/V) C₂H₂ in the first seven incubation days, ranged from 7.61‰ to 21.11‰. Compared with 0.1% (V/V) C₂H₂, the weighted average SP values of N₂O under 95%, 80% and 67% WFPS treatments with 10% (V/V) C₂H₂ produced from soil reduced by 0.10, 0.33 and 0.06 times respectively.【Conclusion】 The increase of soil water content promotes N₂O emission, and the 95% WFPS treatment showed the highest N₂O emissions. In the 67% WFPS treatment, the initial stage of N₂O emission was dominated by denitrification, followed by nitrification. Denitrification was the dominate process in 80% WFPS treatment and nitrification was the dominate process in 95% WFPS treatment.

Key words: water-filled pore space (WFPS), N₂O, nitrification, denitrification, stable isotope, site preference value (SP value)